WatWasWaar: WatWasWaar.nl is a national platform for disclosing historical-geographical information on the Netherlands to a wider audience. The site has several search vectors: Where (location), what (typology), when (time) and which (institute). The site has a nice interface to find things using these vectors. The site doesn't seem to have any user contributed content and the software doesn't seem to be open source. Site is operated by a consortium.

Global World History ATlas A project by Jeff Meyer, collecting together maps resources from all around the world for easy reuse especially in educational environments.

Pleiades is a community-built gazetteer and graph of ancient places. It gives scholars, students and enthusiasts worldwide the ability to use, create, share, and map historical geographic information about the ancient world. It associates names and locations in time and provides structured information about the quality and provenance of these entities. Pleiades is a joint project of the Ancient World Mapping Center, the Stoa Consortium, and the Institute for the Study of the Ancient World.

Pelagios

Limes

Finnish Spatio-Temporal Ontology (SAPO) is an ontology time series of Finnish municipalities over the time interval 1865–2010. Modeling these changes can be done using ontology time-series that represents knowledge about changes (e.g. merges, splits) in historical regions (e.g. cities, counties) and a method to help creating geospatial ontology time-series. Created as a part of the FinnONTO at Computing Research Group (SeCo)